A 500-million-year-old fossil offered a rare treasure: The imprint of an animal that literally died in its tracks.


CreditCreditShuhai Xiao,Virginia Tech

More than half a billion years ago, in what is today southern China, a worm resembling an ear of wheat moved along a stretch of sediment underwater, paused, and left a detailed imprint of its undercarriage in the wet earth. Then it moved a little further and died. Its body, seven inches long and segmented, became a fossil. So did its near-final resting place, creating a mortichnia: a body preserved with its final “death march.”

This was at least 10 million years before the start of the Cambrian Explosion, during which many of the animal groups that exist today appeared, and more than twice as long ago as the appearance of the first dinosaurs. The creature, Yilingia spiciformis — named after the Yiling district in which it was discovered — was a complicated one by the standards of the Ediacaran Period: mobile, segmented, trilobate (each body segment composed of three lobes) and bilaterally symmetrical.

Plants had not yet colonized land, although, according to Rachel Wood, a geoscientist at the University of Edinburgh, there was some “slimy life clinging to the edge of freshwater lakes.”

Yilingia and its death march are the subject of a study published on Wednesday in Nature. The worm is remarkable itself, as is the record of its death. A mortichnia is very rare — the imprint-maker tends to wander off. “It’s like in forensics,” said Shuhai Xiao, a geobiologist at Virginia Tech and one of the authors of the study. “You find a footprint and can probably tell something about the suspect, but you’d rather catch the suspect on camera.”

Dr. Wood, who reviewed the study, compared a mortichnia to the “final clutching, staggering steps” of a human. Better yet, she said, “the fossil and its trackway are large.”

One or two other underwater animals from the Ediacaran Period are known to have displayed some of Yilingia’s traits, the study’s authors noted, including Spriggina, with a horseshoe-shaped head, and Marywadea, which was similar to Spriggina “but with a half-moon shaped head.”

But neither was trilobate, so they weren’t as closely related to life on Earth today. Other trilobites had hardened skeletons and a distinct head, body and tail, whereas Yilingia “

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